A&P I- Chapter 6- Bones

Functions of Bones

support, protection, attachment point, storage, blood cell formation, and horomone production

What do bones protect?

CNS and visceral organs (thorax and upper absominal cavity)

How does skeletal muscle attach to bone?

tendon

What is hematopoiesis?

formation of blood cells in red bone marrow

What is osteocalcin?

regulates insulin release (stimulates pancreas), glucose homeostasis, energy expenditure

What does the embryo skeleton contain

mostly cartilage

Characteristics of cartilage

strength and resilience (return to original shape), high water content, no nerve supply, avascular, surrounded by perichondrium

What is perichondrium?

fibrous connective tissue, vascularized, limits to twisting

Hyaline Cartilage

Most abundant with collagen fibers- Example: Articular cartilage, costal cartilage (ribs to sternum), respiratory cartilage, nasal cartilage

Elastic Cartilage

Contains elastic fibers and some collagen- Example: external ear, epiglottis (above trachea)

Fibrocartilage

contains rows of chondrocytes (secreted cartilage) alternating with thick collagen bands, compressible and great tensile strength- Example: vertebral discs, knee, pubic symphysis

Appositional Growth

laying down new cartilage on old cartilage, cells under perichondrium deposit new matrix on top of old (tree ring), at surface of cartilage tissue

Interstital Growth

cells divide and secrete matrix within pre-existing cartilage, deeper in tissue, causes cartilage to grow in length (cartilage skeleton is the blueprint)

How are bones classified?

location and shape

Axial Skeleton

long axis of body, skull, vertebral column, ribs

Appendicular Skeleton

limbs and the girdles, pectoral and pelvic girdles, arms, legs, vital for movement

Long Bones

longer than they are wide, column shape- Example: arm and leg bones

Short Bones

cube-shaped- Example: bones in wrists and ankles

Sesamoid bones

bones that form in a tendon- Example: patella

Flat bones

thin, flat, curved- Example: sternum, scapulae, ribs, most cranial bones

Irregular bones

anything that does not fit in other categories- Example: vertebrae, os coxa

What are the layers of bones?

Compact (lamellar) bone and spongy (trabecular) bone

Compact (lamellar) bone

looks smooth and solid, no space

Spongy (trabecular) bone

open spaces with needle- like pieces of bone called trabeculae, open space filled with red or yellow marrow

Trabeculae

needle-like bone pieces, found in greatest concentration along lines of stress (where bone is pushed on) to give it more strength

Structure of flat, irregular, and short bones

thin plate of spongy bone covered by compact bone (spongy sandwich), no large cavities for bone marrow

Structure of Long bones

Diaphysis, Epiphysis, Membranes, Vascularization and Innvervation

Diaphysis

bone shaft, composed of compact bone “collar” with internal medullary cavity (contains bone marrow)

Epiphysis

bone ends, composed of compact bone externally and spongy bone internally, covered with articular cartilage, no cavity, location is where one bone articulates with another bone

Membranes

Periosteum and Endosteum

Periosteum

covers external bone surface except at epiphysis!! well vascularized and innervated, outside nerve covering

Endosteum

covers internal bone surfaces, trabeculae in spongy bone, cavities in compact bone, contain osteoprogenitor (generate into bone forming cells) cells

Nutrient artery and vein

serve diaphysis

Epiphyseal artery and vein

serve epiphysis

Osteon (of lamellar or compact bone)

structural unit of compact bone, helps bone withstand pressure and stress (change osteon, change bone function)

Central Canals (of compact bone)

run through center of each osteon, contains nerve and blood vessels

Perforating canals

extending from central canal, connect neighboring osteons and medullary cavity

Interstitial Lamellae

incomplete lamellae found in between complete osteons- Function: fill in gaps between osteons

Circumferential lamellae

found just deep to periosteum, extend around circumference of diaphysis- Function: resists twisting of long bone

Hematopoietic Tissue

tissue to form blood cells

Red bone marrow

produce blood cells, in adult skull, ribs, sternum, clavicles, scapula, vertebrae, and the heads of femur and humerus (only long bones with this)

Where is Red Bone Marrow found in Infants?

all of skeleton is filled with red bone marrow, limited in adult bony, not much growth

Yellow Marrow

in adult medullary cavity of long bones, contains more fat and less blood supply than red marrow, can be converted to red marrow when they hemmorage

4 Cell Types for Bone Growth

Osteoprogenitor (osteogenic cells), osteoblasts, osteocytes, osteoclasts

Osteoprogenitor (osteogenic) cells

stem cells, mitotically active, can differentiate to osteoblasts

Osteoblasts

bone-forming cells, secrete unmineralized matrix (osteoid) that forms bone tissue, secrete matrix until surrounded

What happens to osteoblasts when it is surrounded by matrix?

they mature and transform to osteocytes

What does “unmineralized" matrix mean?

soft, not yet hardened with mineral salts

Osteocyte

mature bone cell, they monitor and maintain bone matrix, respond to mechanical stress and calcium signals- Example: too many salts= too hard, arms communicate with other cell types

Osteoclasts

bone-degrading cells, maintain, remodel, and repair bones, stimulated when blood calcium homeostasis is low, produce collagenase

Collagenase

break down collagen fibers

Chemical Composition of Bone

Organic (cells and osteoid, matrix), and inorganic (mineral salts, calcium phosphate to make collagen fibers stiffer)

Osteomalacia (adults)

calcium deficiency, less mineral salts deposited in bone than normal, causes weak/soft bone, vitamin D deficiency (need for calcium to be absorbed)

Ricket’s (children)

weak/soft bone due to less mineral salts, skeleton is not done growing so permanent bone deformities, bowed legs and misshapen pelvic girdle, can wear a brace

Endochondral ossification

formation of ossified bone by replacement of cartilage with bone, happens below the skull, hyaline used as a blueprint (embryonic skeleton)- as ossified bone is laid done, hyaline cartilage is broken down

Endochondral Ossification Step 1: Formation of a Bone Collar

osteoblasts lay down bone matrix against cartilage surface to form a collar around the diaphysis cartilage to support, formation of primary ossification center (POC- where bone formation begins) after bone collar formation, POC is rigid and filled with cartilage, cells hypertrophy and harden

Endochondral Ossification Step 2: Cavity forms in diaphysis center

cells inside ossification center die off because they are not getting nutrients, cartilage inside POC break down and calcify, creating a cavity, cartilage outside continues to grow and elongates bone, bone collar prevents diaphysis from being bent/crushed

Endochondral Ossification Step 3: Formation of initial spongy bone in diaphysis

the periosteal bud invades cavity, bud contains nutrient artery/vein, nerve fibers, red marrow elements, osteoprogenitor cells, and osteoclasts, osteoblasts secrete matrix around calcified cartilage, initial formation of spongy bone is bone-covered cartilage trabeculae

Endochondral Ossification Step 4: Formation of medullary cavity and elongation of diaphysis

initial spongy bone is broken down by osteoclasts, forms medullary cavity, cartilage remaining in diaphysis is calcified, broken down, and replaces by bone, secondary ossification center forms in epiphyses

Endochondral Ossification Step 5: Secondary ossification continues in epiphyses

Similar to primary ossification except the spongy bone is retained (no medullary cavity), some areas still have some actively growing cartilage

How does a bone grow in length?

intersitital growth in pre-existing tissue, occurs at the epiphyseal plate (cartilage plate between epiphysis and diaphysis)

Process of Growth in Length

1) New cartilage laid down in epiphyseal plate

2) Cartilage cells at center of epiphyseal plate enlarge and cartilage is calcified (die off)

3) Calcified cartilage is broken down by osteoclasts and osteoblasts lay new bone tissue

What happens to bone growth at the end of adolescence?

cartilage production in epiphyseal plate slows and stops (determined by hormones), existing cartilage is calcified and replaced by bone- bone of diaphysis fuses with bone of epiphysis

How does a bone grow in width?

appositional growth occurs at the same time as bone lengthening (can’t have a skinny bone)

Process of bone growth in width

1) Osteoblasts secrete new matrix at the periosteum

2) Osteoclasts break down bone tissue at the endosteum

What difference in activity rate between the cells must there be for growth in width to occur

osteoblast cavity must exceed osteoclast, growth>deterioation

Growth hormone

controls activity at the epiphyseal plate, released by anterior pituitary gland in brain

What effect does growth hormone have on the epiphyseal plate?

when cells are exposed to growth hormone they are more active, hyposecretion = not enough hormone

Estrogen

causes growth spurt at pubery, in high levels it induces epiphyseal plate closure, causes feminization of certain parts of skeleton

Testosterone

Causes “masculinization” of certain parts of skeleton, no growth spurt but no stopping growing

What is the typical age at which males and females stop growing?

21 (M) and 18 (F)

What is involed in bone remodeling?

bone deposition (new osteoid from osteoblasts) and resorption (break down old bone with osteoclasts)

Maintenance of Ca2+ homeostasis

Ca2+ is essential for excitability of body cells (especially neurons and muscle cells), without Ca2+, neurons do not fire and muscle does not contract

Bone Health

Mechanical/gravitational forces activing on bone tissue drive remodeling, strengthens bone exactly where needed

Factors involed in Deposition and Resorption

Parathyroid homorone (PTH - released in response to decreasing blood Ca2+ levels), and Mechanical stress (Wolff’s law: bones will remodel to adapt to mechanical forces that may or may not be places on them)

Effects of increased PTH release

1) number of osteoclasts at bone increase

2) osteoclasts become more active in bone tissue

Once blood Ca2+ returns to normal, PTH release decreases and osteoclast number and rate of activity decreases

If continue low Ca2+ in blood, excessive PTH release and bone breakdown, it eats through the skeleton

What happens when more stress is placed on a bone?

1) the more trabeculae will be found at the location of loading

2) the thicker the compact bone will be at the location of loading

Example- weightlifters need stronger bones, astronauts have no stress on bones so they aren’t as think (prone to fracture)

Steps of Bone Repair

1) a hematoma forms- collection of blood outside of a blood vessel, clear out fracture with phagocytes, chondroblasts lay down fibrocartilage

2) fibrocartilaginous callus forms

3) bony callus forms

4)  bone remodeling occurs